This project is concerned with the development and use of analytical procedures for identification in characterization of gonadotropin receptor sites in the testis and ovary. Methods of gonadotropin labeling, characterization of the labeled tracer, and optimization of receptor assay methods, are evaluated and improved. Computerized methods for analysis of binding data from kinetic and equilibrium experiments, plus corrections for the tracer characteristics, are applied to studies on the properties of gonadotropin receptors. Natives and modified gonadotropins are evaluated for receptor binding activity, and receptor characteristics are analyzed in animal and human tissues. Current studies include charaterization of the human testicular receptors for LH and FSH.